The ozone layer of the Earth atmosphere strongly absorbs ultra violet (UV) radiations in the 300 nm to 200 nm wavelength region. Therefore, below the ozone layer, which is mainly located in the lower portion of the stratosphere from approximately 13 to 40 kilometres above Earth surface, UV radiations from the sun in the 300 nm to 200 nm wavelength region are essentially absent.
To be able to reject the visible wavelengths and to detect the solar blind UV (SBUV) light, wide band-gap materials such as AlGaN, SiC, ZnO have often been used in SBUV detectors. All of these materials are difficult to grow, not compatible with conventional semiconductor processing, and are difficult for integrate with silicon-based readout electronics.
On the other hand, to increase sensitivity of an SBUV detector, an avalanche multiplication is often implemented by using the avalanche photodiode (APD) structure. For most of direct band-gap compound materials, however, multiplication noise also increases at the same rate as the avalanche gain due to the almost unity ratio of the hole to electron impact ionization coefficients.
An APD is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. APDs can be thought of as photodetectors that provide a built-in first stage of gain through avalanche breakdown. By applying a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain (around 100). Avalanche breakdown is a phenomenon that can occur in both insulating and semiconducting materials. It is a form of electric current multiplication that can allow very large currents to flow within materials which are otherwise good insulators. It is a type of electron avalanche. The avalanche process occurs when the carriers in the transition region are accelerated by the electric field to energies sufficient to free electron-hole pairs via collisions with bond electrons.